In a fuel cell system disclosed in JP 2009-123550 A, a cathode compressor is operated so that a flow rate of the air supplied to a fuel cell stack reaches a target flow rate required for electric power generation. Then, when the cathode compressor supplies the air at an air flow rate (air flow rate for surge avoidance) more than that required for the electric power generation, the compressor supplies the flow rate more than the air flow rate required for the electric power generation, and uses a bypass valve to supply a bypass passage with a flow rate component unnecessary for the stack.